# Engineers Corner > Electrical Engineering electrical

## lakshmi.rose

in a single phase induction motor if the number of poles is 4 frequency is 50hz then what is the speed (RPM) of the motor?

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## krkrishnareddy

speed= 120f/p
        =120X50/4=1500 rpm

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## chyanakyaa

n=120f/p simple.......

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## zabeer

practically i feed 50 hz frequency, for 4 pole induction machine, but i not get 1500 instead i get 1440 RPM, can you please tell, why the droping of rpm

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## Raman Jain

Becasue of slip. if it runs on 1500 rpm it will become syn. motor . It is the property of induction motor.

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## josenit1787

well,n=120*f/p gives the speed of the rotating mmf or the synchronous speed.An induction moor will develop zero torque at this speed and hence will always run at a lower speed(s*n)where s is called the slip.(s is usually in the range .02 to .08 and depends on the load)

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## vinodipti

f=PNs/120
NS=120f/P
    =120*50/4
    =1500 rpm

Nr= Rotor Speed will depend on slip.

Nr= Ns(1-s)

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## anithadharani2007

how can i select the cable for amps rating

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## jeet29

that u can calculate simply by the formulas:

Ns=120*f/p     where Ns=speed of the stator ,f =frequency of supply,
                          and p=no of poles

slip(S)=(Ns-Nr)/Ns

nd rotor frequency fr=S*supply frequency

    with the help o fthese formulas you can get wat u want....

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## jeet29

cable size primarily depends on its voltage rating or kW 
rating of load. From the kW rating, we can calculate the 
current that should be carried by cable and the number of 
runs required. we also have to calculate voltage drop 
during starting and running in case of a motor load.

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## ritesh1981

> practically i feed 50 hz frequency, for 4 pole induction machine, but i not get 1500 instead i get 1440 RPM, can you please tell, why the droping of rpm


all things in practice is  not ideal,though F=50 Hz. I.M doesnot rotate at 1500RPM by120F/P formula b'use  if speed is 1500RPM r,rotor speed =1500RPM,Synchronous flux rotate at 1500RPM ,and EMF induced in the Rotor is zero due to SAME speed hence no induced emf.,no force and Motor will not run.hence slip=Syn.speed-Actual speed =Somthing value   Must for rotating motor.

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